Conventionally, as for the video camera, a contrast emphasis method by way of gradation change, a high frequency component emphasis method of emphasizing contrast of a high frequency component in an image, etc. have been proposed as a method of improving contrast (a difference between brightness and darkness) and a degree of sharpness (bordering precision) of an image taken by imaging devices, such as CCD (Charge Coupled Device), CMOS (Complementary Metal-Oxide Semiconductor), etc.
As the contrast emphasis method, a tone curve adjustment in which a pixel level for each pixel of an image is converted with a function (hereafter referred to as a level conversion function) having a predetermined input-and-output relationship, and a method referred to as histogram equalization in which the level conversion function is adaptively changed according to frequency distribution of pixel levels have been proposed.
As the high frequency component emphasis method, a method referred to as an unsharp mask has been proposed that performs a so-called edge enhancement in which an edge is extracted from the image and the extracted edge is emphasized.
In the contrast emphasis method, however, there is a problem in that only some bright regions within all the dynamic range (a difference between the maximum level and the minimum level) of the image can be increased in contrast. In addition, there is another problem in that the contrast is instead reduced in the brightest part and the darkest part of an image in the case of the tone curve adjustment, and near a luminescence region with low frequency distribution in the case of the histogram equalization. Furthermore, in the high frequency component emphasis method there is another problem in that only the contrast of the high frequency component of the image is emphasized, whereby the portion near the edges of the image is unnaturally emphasized, and deterioration of image quality is unavoidable.
Then, there is a conventional method in which, in a situation where edges having a steep change in a pixel value among input image data are saved, portions other than the edges are amplified with an image signal processing apparatus constructed as shown in FIG. 1, to thereby emphasize the portions other than the edges (for example, Japanese Patent Application Publication (KOKAI) No. 2001-298621).
In the image signal processing apparatus as shown in FIG. 1, an inputted image signal is inputted into an ε filter 1 and a subtraction unit 2. The ε filter 1 receives, as an input, an image signal slightly changing on both sides of a steep edge as shown in FIG. 2A, converts it into an image signal in which only edges as shown in FIG. 2B are extracted, and which is outputted to the subtraction unit 2 and an adder 4.
Particular processing of the ε filter 1 will be described with reference to FIGS. 3 and 4. The ε filter 1 determines each pixel of the input image to be a pixel of attention C one by one. As shown in FIG. 3, taps are set up, including a plurality of neighbouring pixels (in this case six pixels L3, L2, L1, R1, R2, R3) which are horizontally successive and centered around the pixel of attention C. As shown in the following expression (1), pixel values of the pixel of attention C and the plurality of neighbouring pixels are subjected to weighted averaging by means of the tap coefficients (for example, {1, 2, 3, 4, 3, 2, 1}), and outputted as a conversion result C′ corresponding to the pixel of attention C.C′=(1·L3+2·L2+3·L1+4C+3·R1+2·R2+1·R3)/16  (1)
However, as shown in FIG. 4, a neighbouring pixel (in the case of FIG. 4, neighbouring pixels R2 and R3) having a difference, which is larger than a predetermined threshold value ε, between its pixel value and the pixel of attention C is calculated by replacing it by that of the pixel of attention C. That is, in the case of FIG. 4, the following expression (2) is calculated.C′=(1·L3+2·L2+3·L1+4·C+3·R1+2·C+1·C)/16  (2)
Now, returning to FIG. 1, the subtraction unit 2 subtracts the image signal inputted from the ε filter 1, from the image signal (the same as the input to the ε filter 1) inputted from the preceding stage, extracts the image signal slightly changing other than that of the edge, and outputs it to an amplification unit 3. The amplification unit 3 amplifies the output of the subtraction unit 2, and outputs it to the adder 4. The adder 4 adds the image signal in which parts other than the edge outputted from the amplification unit 3 are amplified, to the image signal in which only the edge inputted from the ε filter 1 is extracted. This produced total is the image signal in which the parts other than the edge are amplified in the situation where the steep edge is held.
Incidentally, in the ε filter 1 of the image signal processing apparatus as shown in FIG. 1, when an image signal having a larger edge size than the predetermined threshold value ε is inputted as shown in FIG. 5 for example, the image signal after conversion is such that the phase is shifted to the left-hand side, as shown in FIG. 6. That is, there is a problem in that the edge with the steep change in pixel value is not held correctly, and an image quality may deteriorate.